The present invention is generally related to voltage transformers. More specifically, the present invention is related to detecting the presence of a partial discharge in a voltage transformer.
Voltage transformers transfer AC voltage and current to different levels at essentially constant power and frequency. The power transfer is performed within a coil comprised of two voltage windings, i.e., circuits, operating at different voltages. Insulation can be placed between adjacent windings. The type and size of insulation is dependent upon the designed electric field stress. Depending upon the type of transformer, part of the voltage winding can be common to both circuits.
The transformer is constructed in a shell or core structure. In its simplest form, two voltage windings are operably linked by a magnetic core. The windings are comprised of a conductor sheet or a conductor wire having a circular or rectangular cross-section. Conductive materials such as copper, aluminum, etc. are used to construct the voltage windings. The windings can be configured for single-phase and three-phase operation, i.e., delta (xcex94) and wye (Y) connections.
The performance life of a transformer is largely related to its insulation system and its operating temperature. Generally, the better the cooling system provided for the transformer, the higher kVA rating that can be designed. Large transformers utilize insulated fluid ducts to provide a path for cooling fluid to flow within the transformer. Some known cooling systems incorporate either air, oil, water or a combination thereof as the cooling fluid.
Other factors also affect the life of the transformer. Such factors include: vibration, insulation type, mechanical stress, repetitive expansion and contraction, exposure to moisture and other contaminants and electrical and mechanical stress associated with over-voltage and short-circuit currents.
Another factor affecting a transformer""s performance is the occurrence of a partial discharge in the transformer. Partial discharge within a voltage transformer is an electric discharge that only partially bridges the insulation between conductors. The term xe2x80x9ccoronaxe2x80x9d has been frequently used with this connotation. The partial discharge causes insulation deterioration within the transformer and is often a reason for the eventual breakdown of the insulation system.
Presently, partial discharge is not easily detected outside of a controlled environment, such as a laboratory. Typically, a remote power source is implemented to induce a voltage in the transformer to be tested. A capacitor divider circuit provides a voltage tap on the transformer for connection to a radio frequency detector. The radio frequency detector monitors the transformer for the occurrence of a partial discharge. Because of the large test voltage, this type of partial discharge detector is susceptible to electrical noise hampering its ability to detect a partial discharge.
Another detection technique includes providing a sensor, i.e., antenna, within the coil structure for detecting electrostatic discharges.
In addition to being ineffective at detecting the occurrence of a partial discharge, these techniques are not easily implemented in the field.
This invention is directed to solving these and other problems.
The present invention is directed to detecting the occurrence of a partial discharge in a voltage transformer. More specifically, the present invention provides for detecting a partial discharge in a voltage transformer operating in a field environment.
According to an embodiment of the present invention, a detector system for detecting partial discharge in a voltage transformer having a first and second voltage winding comprises an electrostatic shield operably connected to the transformer. A radio frequency current transformer is operably connected to the electrostatic shield wherein a partial discharge occurring in the voltage transformer is identified by the electrostatic shield and the radio frequency current transformer.
Preferably the electrostatic shield is located between the voltage windings. The electrostatic shield substantially surrounds the first voltage winding and is located between the first and second voltage windings. The electrostatic shield is electrically grounded and insulated from the windings. The insulator comprises a glass mat impregnated with epoxy. A partial discharge occurring within the transformer is sensed by the electrostatic shield and identified by the radio frequency current transformer. More specifically, a ground current resulting from the occurrence of a partial discharge, flows through the electrostatic shield. The generated ground current is identified by the radio frequency current transformer.
Another embodiment of the present invention is directed to the electrostatic shield being operably attached to the transformer and substantially surrounding the second voltage winding. The electrostatic shield is operably connected with a conductor lead to electrical ground. A radio frequency current transformer is operably connected to the conductor lead of the electrostatic shield. A ground current generated by the partial discharge in the voltage transformer is sensed by the electrostatic shield and identified by the radio frequency current transformer. Operator personnel can monitor the occurrence of a partial discharge in the voltage transformer by observing the radio frequency current transformer.
An object of the present invention is to provide a low cost, dependable, partial discharge detector integral with a field deployed voltage transformer.
Another object of the present invention is to provide a reliable detector of a high frequency ground current signal generated by a partial discharge occurring within a voltage transformer.
A further object of the present invention is to provide a signal having an increased signal-to-noise ratio to represent the detection of a partial discharge occurring in a voltage transformer.
Other advantages and aspects of the present invention will become apparent upon reading the following description of the drawings and detailed description of the invention.